This invention relates to an internal combustion engine and more particularly to an improved construction for and method of operating such an engine to improve its performance.
As is well known, a wide variety of expedients have been resorted to in order to improve the performance of internal combustion engines. Because of the wide variety of loads and speeds under which such engines are required to operate for many applications, a great number of systems have been proposed to improve the performance under these widely varying conditions. Such devices as variable intake systems, variable valve timing, exhaust gas recirculation, feed back control and fuel injection have been employed in order to improve engine performance.
Common with most of these control strategies and systems, is the desire to maintain the engine operating in a relatively lean state under conditions when maximum power output is not required. So called "lean burn" engines have been proposed and a number of expedients have been employed for achieving good running when operating under leaner than stoichiometric fuel/air ratios.
One methodology employed to control exhaust emission is to utilize exhaust gas recirculation. The exhaust gases are recirculated from the combustion chamber back into the combustion chamber in a wide variety of manners. By doing this it is possible to control certain unwanted exhaust emissions such as nitrous oxides (NOx). However, lean burn engines have a fairly high sensitivity to the amount of exhaust gas recirculation that can be employed while still maintaining stable combustion. Thus, the combination of lean burn and exhaust gas recirculation presents substantial problems for the designer.
One way of providing exhaust gas recirculation is by operating the intake and exhaust valves in such a manner that a portion of the exhaust gases actually pass into the intake passage during the valve overlap. When this is done, the fresh charge that enters the combustion chamber through the intake passage will be partially diluted by the exhaust products that have passed into the intake system when the intake valve is first open. This is a very effective and simple way of obtaining exhaust gas recirculation and is referred to as "internal EGR". However, it is very difficult to achieve the desired control and maintain good engine performance.
It is, therefore, a principal object of this invention to provide an improved engine combustion and control system embodying internal exhaust gas recirculation.
It is a further object of this invention to provide an improved arrangement for control the amount of internal exhaust gas recirculation under running conditions and particularly idle and low load and speed range conditions.
In accordance with the invention it has been discovered that the amount of internal EGR can be very effectively controlled by controlling the relationship between the intake and exhaust valve overlap and the ratio of the volume of the induction passage between the throttle valve and the intake valve port and the volume of the combustion chamber. This ratio, hereinafter referred to as the "volume ratio" may be expressed as the ratio between the port volume, i.e., the volume of the intake passage between the throttle valve and the intake port and the exhaust gas volume, that is the maximum volume of the combustion chamber at bottom dead center position of the piston including the clearance volume.
It is, therefore, a further object of this invention to provide an improved engine combustion system and operating system wherein the volume ratio is maintained at a desired relationship relative the valve overlap in order to obtain the appropriate amount of EGR and to permit an extension of the lean running limit for good and efficient combustion.